The invention describes a method for producing particle structures using magnetic fields, and more particularly, a method for producing particle structures from a suspension of particles using a triaxial magnetic field.
There is considerable interest in particle composite materials with definable and controllable two- and tree-dimensional structures, which have applications such as structural materials, composite magnets and dielectrics, conducting adhesives, current-limiting thermistors, and recently, sensors. One of the main difficulties in designing particle composites is optimally organizing the particles for a particular purpose. In a typical composite, the particles are simply randomly dispersed, though some aggregation typically occurs before matrix polymerization. Some degree of structural control can be achieved if the particles have a significant permeability or permittivity mismatch with the matrix. Application of a uniaxial magnetic or electric field to such suspensions will organize such particles into one-dimensional chains, producing uniaxial Field-Structured Composites (FSCs). It is also possible to organize particles into two-dimensional sheets by subjecting a suspension to a biaxial field, such as an audio-frequency rotating field, thus producing biaxial FSCs. These composites have highly anisotropic magnetic and dielectric properties, and enhanced magnetostriction and electrostriction coefficients.
Field-structured materials are a recently-discovered class of materials possessing a substructure of ordered aggregates of suspended dielectric or magnetic particles. This substructure self-assembles under the influence of an external field, such as an external electric or magnetic field, and induces a wide range of mechanical, dielectric, magnetic, and optical properties. The mechanism of structure formation can be described as follows: When a magnetic particle suspension, consisting of multi-domain particles, is exposed to a uniaxial magnetic field, the magnetic dipole moment on the particles will generally increase and align with the applied field. The particles will then migrate under the influence of the dipolar interactions with neighboring particles, to form complex chain-like structures. If a magnetic particle suspension is instead exposed to a biaxial (for example, rotating) magnetic field, the induced dipole moments create a net attractive interaction in the plane of the field, resulting in the formation of complex sheet-like structures. Similar effects occur when suspensions of dielectric particles are subjected to uniaxial and biaxial electric fields. These materials can have large anisotropies in such properties as their conductivity, permittivity, dielectric breakdown strength, and optical transmittance.
A method to make isotropic structures that have enhanced properties in three dimensions (for example, orthogonal directions) could offer significant advantages over methods utilizing uniaxial or biaxial magnetic field methods, which can produce materials with enhanced properties in only one or two directions, respectively. Isotropic structures are not produced using uniaxial or biaxial fields, and it would seem that the use of a triaxial field would result in the cancellation of dipolar interactions that could lead to such three-dimensional structures.